Printed circuit boards are formed from a layer of conductive material (commonly, copper or copper plated with solder or gold) carried on a substrate of insulating material (commonly glass-fiber-reinforced epoxy resin). A printed circuit board having two conductive surfaces positioned on opposite sides of a single insulating layer is known as a "double-sided circuit board." To accommodate even more circuits on a single board, several copper layers are sandwiched between boards or other layers of insulating material to produce a multi-layer circuit board.
To make electrical connections between the circuits on opposite sides of a double-sided circuit board, a hole is first drilled through the two conducting sheets and the insulator board. These holes are known in the art as "through holes." Through holes are typically from about 0.05 mm to about 5 mm in diameter and from about 0.025 mm to about 6 mm. long. The through hole initially has a nonconductive cylindrical bore communicating between the two conductive surfaces on opposite sides of the board. A conductive material or element is positioned in the through hole and electrically connected with the conducting sheets on either side of the through hole.
Like double-sided circuit boards, multi-layer circuit boards also use holes in an intervening insulating layer to complete circuits between the circuit patterns on opposite side of the insulating layer. Unless the context indicates otherwise, references in this specification to "through holes" refer to these holes in multilayer boards as well, even if they do not literally go through the entire circuit board.
Various conductive elements have been devised over the years for forming a conductive pathway via the through hole. Initially, conductive solid parts (e.g., rivets or eyelets) were inserted through the through holes and mechanically secured in place. However, these parts were labor intensive to install and proved unreliable with age. Jumper wires running around the edge of or through the board and the leads of conductive elements soldered to the board have also been used.
More recently, conductive material--typically, a layer of copper--has been coated on the nonconductive through hole bore to provide a cylindrical bridge between the conducting sheets which lie at the opposite ends of the through hole. Electroplating is a desirable method of depositing copper and other conductive metals on a surface, but electroplating cannot be used to coat a nonconductive surface, such as an untreated through hole. It has thus been necessary to treat the through hole with a conductive material to make it amenable to electroplating.
One process for making the through hole bores electrically conductive, to enable electroplating, is to physically coat them with a conductive film. The coated through holes are conductive enough to electroplate, but typically are not conductive and sturdy enough to form the permanent electrical connection between the conductive layers at either end of the through hole. The coated through holes are then electroplated to provide a permanent connection. Electroplating lowers the resistance of the through hole bore to a negligible level which will not consume an appreciable amount of power or alter circuit characteristics.
Conductive through hole coating compositions containing nonmetallic, electrically conductive particles have long been sought to avoid the expense and disposal problems associated with metal deposition. The only common nonmetallic conductors are graphite and carbon black. Of these two, graphite is far more conductive, so the art has long sought to make a graphite dispersion which is suitable for coating a through hole with a conductive layer of graphite. Graphite dispersions, however, have been found unsuitable for preparing through holes for electroplating.
U.S. Pat. No. 3,163,588 (Shortt), which issued on Dec. 29, 1964, briefly suggests that a through hole surface may be rendered conductive prior to electroplating by applying a paint or ink containing a substance such as graphite. Col. 3, ln. 57-58.
U.S. Pat. No. 3,099,605 (Radovsky), which issued on Jul. 30, 1963, states, however, that the prior use of graphite to form a conductive base coating on the exposed areas of a through hole suffered from many "defects." Col. 1, ln. 66. These defects were said to include the "lack of control of the graphite application with the resultant poor deposit of the electroplated metal and non-uniform through hole diameters." Col. 1, ln. 66-70.
U.S. Pat. No. 4,619,741 (Minten) teaches that "when graphite particles are used . . . loss of adhesion of the copper to the non-conducting material after the subsequent electroplating was noted." Col. 7, ln. 11-16. In comparison 1 of the '741 patent, through holes that were electroplated after application of the first substitute graphite formulation (2.5% by weight graphite) had only a few visible voids, "but failed the solder shock test." Col. 20, ln. 5-7. According to the '741 patent, "[t]he plated on copper in the holes pulled away from the epoxy/glass fiber layer." Col. 20, ln. 7-8. The results were even worse with the second substitute graphite formulation (0.5% by weight graphite). After electroplating, the boards that were treated with the second substitute formulation had voided holes. See: col. 20, ln. 14. According to the '741 patent, "[t]he standard shock test could not be run on boards that were prepared with this latter graphite formulation because of the lack of unvoided holes." Col. 20, ln. 14-16.
U.S. Pat. No. 5,139,642 (Randolph), which issued on Aug. 18, 1992, contains comparative examples 3A and 3B in which a graphite dispersion was coated in a single pass on a through hole and dried to form a graphite layer directly on the nonconductive substrate. The substrate was then subjected to a through hole electroplating process. The test was a failure: the patent states that "[t]his board (C-3B) was not evaluated for adhesion since significant voids were observed even after 55 mins. of plating."
A competing process for plating through holes has been to use electroless copper--a solution which plates metal through chemical action, requiring no electricity, and which thus will deposit conductive metal on a nonconductive substrate. Electroless copper can plate copper directly on the through hole to make it conductive. Then, typically, electroplating is used to build up the coating, providing a permanent conductive path.
U.S. Pat. No. 4,619,741 (Minten), which issued on Nov. 11, 1986, teaches that, since about 1961, the industry had relied upon electroless copper deposition to prepare the walls of a through hole
for electroplating. Col. 1, ln. 25-28. Although electroless deposition provided superior results to the prior art methods for preparing a through hole surface, electroless deposition has several commercial disadvantages. As pointed out by Minten, these disadvantages include a six step process prior to electroplating; a long process time; multiple treatment baths; a "complex chemistry which may require constant monitoring and individual ingredients which may require separate replenishment;" a "palladium/tin activator [which] also may require extensive waste treatment;" and a "multiplicity of rinse baths [which] may require large amounts of water." Col. 1, ln. 66, to col. 2, ln. 7.
Radovsky, cited previously, nonetheless states that the electroless plating method "has advantages over the graphite methods."Col. 2, ln. 10-12. "[The] advantages are essentially better control over the base layer of catalyst metal deposition and a resultant improved electroplating process with more uniform hole diameters." Col. 2, ln. 12-15.
To overcome the disadvantages associated with the electroless and graphite deposition methods, U.S. Pat. 4,619,741 (Minten), cited above, teaches coating the non-conductive surface of a through hole wall of a printed circuit board with carbon black particles prior to electroplating. The '741 patent expressly teaches that "graphite particles" are not capable of substituting for the carbon black particles. According to the '741 patent, "both graphite formulations were far inferior for electroplating preparation as compared to the above carbon black formulations." Col. 20, ln. 17-19.
The following U.S. patents also teach that graphite is not a substitute for carbon black in carbon black formulations that conductively coat through holes prior to electroplating: U.S. Pat. No. 4,622,108 (Polakovic: one of the present inventors) at col. 8, ln. 1-5; U.S. Pat. No. 4,631,117 (Minten) at col. 7, ln. 24-28 ("when graphite particles are used as a replacement for the carbon black particles of this invention, the undesirable plating characteristics mentioned in U.S. Pat. No. 3,099,608 would likely occur"); U.S. Pat. No. 4,718,993 (Cupta) at col. 8, ln. 27-37; and U.S. Pat. No. 4,874,477 (Pendleton) at col. 7, ln. 60-68.
In addition, the following U.S. patents discuss the deficiencies associated with using graphite as a conductive coating prior to electroplating: U.S. Pat. Nos. 4,619,741 at col. 2, ln. 16-25; 4,622,108 at col. 2, ln. 12-20; 4,622,107 at col. 1, ln. 52-60; 4,631,117 at col. 2, ln. 22-30; 4,718,993 at col. 2, ln. 21-29; 4,874,477 at col. 1, ln. 54-62; 4,897,164 at col. 1, ln. 54-62; 4,964,959 at col. 1, ln. 28-36; 5,015,339 at col. 1, ln. 56-64; 5,106,537 at col. 1, ln. 34-42; and 5,110,355 at col. 1, ln. 60-68. According to these patents, the deficiencies with the graphite process included lack of control of the graphite application, poor deposit of the resultant electroplated metal, non-uniform through hole diameters, and high electrical resistance of the graphite.
The carbon black process is commercially available under the BLACKHOLE trademark from MacDermid Incorporated of Waterbury, Conn. It is difficult to make the BLACKHOLE process work, however, and it provides a coating with an undesirably high electrical resistance. All the current used for electroplating must flow through the carbon black coating, so, for a given voltage, the current flow through a high resistance coating is relatively low. The rate of electroplating is proportional to the current flow, so a high resistance coating requires a long plating time to plate the desired quantity of metal over the carbon black coating. The voltage drop across the high resistance coating also consumes electricity by generating heat.
The electrical resistivity problem with the carbon black process has been addressed commercially in the BLACKHOLE process by depositing a second coat of carbon black over the first to further lower the resistivity of the coating. Of course, this two-pass process requires more materials, time, and equipment than a one-pass process.
The Randolph patent cited previously teaches that the deficiencies of a single graphite layer or a single carbon black layer can be avoided by applying an aqueous dispersion of carbon black directly to the through hole, removing the water to leave a carbon black film, then applying an aqueous dispersion of graphite to the carbon black film, and finally removing the water to form a second, graphite film. The carbon black film acts as a primer for the graphite film to increase adhesion, while the graphite layer is more electrically conductive and thus lowers the resistivity of the composite coating. But a two-pass process is again required.